Low density mist collector pad

ABSTRACT

A wire mesh mist collector pad having multiple density zones, the zones being arranged other than to provide a constantly increasing density gradient, provides a reduced pressure drop and increased capacity. For example, the pad can include at least three zones with the zones arranged so that the density gradient in the direction of gas flow through the pad varies from low to high to low or from high to low to high.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. application Ser.No. 12/728,909 filed on Mar. 22, 2010, which is a continuation of U.S.application Ser. No. 11/446,656 filed Jun. 5, 2006. The contents of U.S.applications Ser. Nos. 12/728,909 and 11/446,656 are hereby incorporatedby reference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to improved mesh for removing entrained orsuspended droplets or particulates in a gas stream.

2. The State of the Art

Many chemical and other industrial processes create a gas stream thatincludes solid and/or liquid particles that must be removed from the gasstream. These particles are typically liquid droplets created byentrainment, impingement, chemical reaction, or condensation, but canalso be solid particles.

Perhaps the most typical of methods for removing these unwantedparticles is a device which causes the gas/particle flow to changedirection. Because the particle has a higher density than the gas of thestream, the momentum of the particle will tend to make the particletravel in a straight line and not change direction as quickly as thegas. For example, in the case of a stream of water droplets in air, thegas stream can be passed through a wire mesh: the liquid droplets cannotnegotiate the tortuous path through the mesh and so they land on andadhere to the mesh, and coalesce, by surface tension, and then run offfrom the mesh due to gravity. Gas-liquid separations that use demistingpads include gas plants, refineries, steam-generating power plants, gasscrubbers, and various other operations used in producing petrochemicalsand speciality chemicals.

Another device typically used for removing liquid or solid particlesfrom a gas stream includes a series of wave plates (vanes) arrangedparallely, each vane being a thin sheet that is formed into hills andvalleys. The vanes are arranged spaced closely together. The gas streamenters one side and takes a zig-zag path to reach the other side. Theentrained droplets cannot negotiate the rapid zig-zag and impinge on thevane, where they cling and run down the wall. While vanes are morerobust, they are significantly heavier and more costly to manufacturethan a wire mesh mist collector.

The typical industry standard for mesh pads is to use 0.011 inch wire ata density of 9 lb./ft³. The wire commonly used is 304SS, or any suitablealloy, metal, or plastic chemically compatible with the process stream.Some operations use a multilayered pad having the lowest density padupstream and the highest density pad downstream. Although termed“multilayered,” the pad actually has multiple zones, where each zone iscomprised of one or more layers of the same mesh. Multilayer padspresently used in the industry have two or three zones at most. The padmust remove entrained material but without causing any significantpressure drop. In a typical plant, a pressure drop of one inch of wateracross a pad amounts to hundreds of thousands of dollars a year in a onebillion SCF/d plant to push the gas around with the increased pressuredrop.

SUMMARY OF THE INVENTION

In light of the foregoing, among various objects of this invention areto make a pad that has good if not improved capacity, to make the padlighter and so use less material, thereby making the pad less costly tomanufacture, and to provide a lower pressure drop across the pad,thereby reducing the operating cost of the processing facility, andespecially providing all of these benefits in a single device.

In summary, this invention provides a multizone mesh pad having a lowerdensity and a higher capacity than existing pads, and which is made witha smaller diameter wire. The pad is made by knitting the wire into atube, flattening the tube in one direction, then crimping the tube in adifferent direction, optionally crimping the tube in yet a differentorientation, forming cut lengths of various crimped mesh tubes into apad by stacking the lengths in a desired order to having alternatinghigh and low density zones, and fixing the pad to a grid forinstallation in process equipment.

In a particular embodiment, this invention provides a multilayer padhaving at least three zones of alternating high and low density mesh,each zone comprising at least one layer of mesh, more preferably atleast four zones, even more preferably at least five zones, and mostpreferably at least six zones. The mist collector pads of this inventiondo not have an increasing density gradient across the entire pad, butinstead alternate the density gradient.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-10 depict results of testing the multilayer pad of thisinvention against other separation devices, with the comparativepressure drop data shown in odd-numbered figures and comparativeentrainment data shown in even-numbered figures.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The mesh used to make the pads of this invention is made with aconventional knitting machine that knits wire instead of thread.Although it is preferred in this invention to use a thinner wire, suchas 0.006″ as compared with the 0.011″ wire conventionally used in theindustry, the advantages obtained by alternating high and low densitymeshes to make a multilayer pad provides improved capacity and a lowerpressure drop. The wire must be chosen to be inert in the environment ofuse, and type 304SS or 304L is typically used when droplets are removedfrom an aqueous or organic stream; 316L, Alloy 20, andpolytetrafluoroethylene-based polymers (TEFLON) are used for sulfuricacid; 410SS and glass are used for mild chemicals; a nickel-copper alloy(MONEL) is used for corrosive chemicals, nickel is used for caustic; andthe like as is known in the industry.

Multilayer pads in the prior art have a density gradient increasing inthe direction of flow; for example, a lower density pad upstream and ahigher density pad downstream. Surprising it has been found that simplyvarying the density between adjacent layers of a multilayer pad providesimproved capacity and a lower pressure drop. Thus, alternating from highto low to high, or low to high to low, provides improvements. As shownbelow in Example 4, in comparison with a pad having the same totalthickness using meshes of lower density (5 lb/ft³ and 9 lb/ft³) than theembodiment of the invention in Example 4 (9 lb/ft³ and 12 lb/ft³ meshdensities), the present invention provides a lower pressure drop and hasa higher capacity.

The pads of this invention are first made by knitting a wire, such as0.006″ type 304SS wire, on a conventional knitting machine to produce aknitted tube or sock. The knitted tube is first flattened by being runbetween roller. The flattened pad is then crimped by being run betweentwo rollers, at least one having a pattern (like an embossing pattern)thereon so that the mesh tube is flattened and crimped. The crimped meshtube may optionally then be crimped in a different direction ororientation between a second pair of rollers, having the same or adifferent pattern, the rollers oriented differently with respect to themesh than the first pair. For example, if the first pair of crimpingrollers is disposed horizontally, the mesh is fed horizontally; then,without changing the orientation of the mesh, it is run between a secondpair of crimping rollers disposed vertically. The two pairs of crimpingrollers need not be orthogonal to each other, although that ispreferred; it is enough that the orientation of the second pair isdifferent than the orientation of the first pair with respect to a giventube orientation. The resulting tube has a lower density than theoriginal knitted and uncrimped tube. By varying the number of wires usedto knit the tube and the number of layers of mesh used to make a givenlayer in the mist collector, as well as the number of crimpingoperations, a layer having a desired density can be produced. Increasedcrimping lowers the final density of the mesh. In the examplesfollowing, one mesh style (3BF) is made from 0.006″ wire into a meshlayer having a density of about 7.2 lb/ft³ (120 ft²/ft³; 98.6% voids),and another mesh style (3BA) is made with the same wire diameter into amesh layer having a density of 12.0 lb/ft³ (200 ft²/ft³; 97.6% voids).

The mesh layers are then layered or stacked in a desired order on aframe or grid that both supports the multilayer pad and connects it tothe process vessel (and holds it in place against the gas flow). Thegrid is secured and the mist collector is then installed into theprocess equipment.

EXAMPLES

A sample multilayer pad according to this invention was compared with aconventional multilayer pad on a testing apparatus. In the testingapparatus, a mixture of air and water was used as the test stream, usinga 15 HP radial blade blower with an inlet damper for air flow control, a12 inch., Sch. 20, 16 foot long exit pipe from the blower, and using aDwyer model DS-400-12 multi-orifice flow sensor for air flowmeasurement, all for supplying a vertical test chamber. The outlet fromthe test chamber had a 40 inch long Sch. 20 pipe including a FilterSenseModel LM-70 Mist Gauge for entrainment measurement. The pressure dropwas measured with an inclined manometer (measurement in inches of watercolumn), the test system temperature having been measured with a WekslerInstruments dial thermometer with 1° F. gradations. The inlet loadingwas applied with either full cone water (Bete SCM9SQ), but morepreferably, as in these examples, using a two-fluid (Spraying Systems1/2J+SU 79) air-water spray nozzle. The water was recycled to the inletusing a 2HP Teel centrifugal pump and metered using a zero to five GPMrotameter. The multilayer pad devices were tested to determine theircapacity to eliminate water drops from the air stream. Capacity wasdetermined by the air velocity at which breakthrough started to occur.The amount of water in the exit stream was determined using an electricinduction probe (model LM 30, ProFlow brand series, from ImpolitEnvironmental Control Corp., Beverly, Mass.). The outputs of the probewere used to calculate entrainment ENTR as gal_(water)/mmSCF_(air)(gallons of water per million standard cubic feet of air).

The sample pad (designated below as “LDP”) was constructed by layeringthe aforementioned style 3BA and 3BF meshes as follows, all being madewith 0.006″ 304SS wire, to make the pad designated as “LDP” herein, allof the 3BF layers having been crimped twice and the 3BA layers havingbeen crimped once. A “layer” in the pad construction is a single crimpedknit tube. The sample LDP had the following construction (the directionof gas flow being bottom to top):

TOP   6 layers of 3BF   1 layer of 3BA   2 layers of 3BF   1 layer of3BA   2 layers of 3BF   1 layer of 3BA   3 layers of 3BF   1 layer of3BA   6 layers of 3BF BOTTOMThe direction of the gas flow in the testing apparatus is verticallyfrom bottom to top. The total thickness of the multilayer pad was sixinches and the average density was 2.9 lb/ft³.

Example 1

The LDP mist collector pad was tested against a six inch thick style 4CApad, made with 0.011″ wire, having a density of 9 lb/ft³, 85 ft²/ft³surface area, and 98.2% voids, layered to provide a thickness of sixinches. As shown in FIG. 1, the LDP pad has significantly less pressuredrop over the entire air velocity range, and FIG. 2 shows thatbreakthrough on the 4CA pad occurred at about 19 ft is air velocitywhereas the LDP pad experienced breakthrough at about 23 ft/s.

Example 2

An eight inch comparison device was made (upstream to downstream) usingfive inches of a conventional vane separator (VH12), one inch of 7CAmesh, and two inches of 3BF style mesh. The style 7CA mesh is made from0.011″ wire, has a density of 5.0 lb/ft³, a surface of 45 ft²/ft³, and99.0% voids. This VH127CA3BF pad was tested against the LDP pad of thisinvention. As shown in FIG. 3, the LDP pad had a lower pressure dropthroughout the air velocity range, and FIG. 4 shows the pads havecomparable breakthrough, yet the LDP pad is only three-quarters thethickness of the comparison pad.

Example 3

A six inch pad (2″7CA4″4CA) was made with two inches of 7CA style meshand four inches of style 4CA mesh to produce a six inch pad. This wastested against the six inch LDP pad. As shown in FIG. 5, the LDP pad hada lower pressure drop through the flow range, and FIG. 6 shows thatbreakthrough occurred at a higher air velocity in the LDP pad than withthe comparison pad.

Example 4

The present six inch LDP pad was tested against eight inches of standardundulating vanes with a spacing of one-half inch between vanes. As shownin FIG. 7, the pressure drop was slightly better for the vanes than theLDP pad, although FIG. 8 shows that breakthrough occurred at a higherair velocity using the LDP pad than the vanes.

Example 5

The present LDP pad was tested against an eight inch pad made using(upstream to downstream) one inch of the 7CA style mesh, two inches of3BF style mesh, and five inches of the conventional vane (similar toExample 3 but reordered). FIG. 9 shows that the novel LDP pad exhibiteda lower pressure drop over the entire air flow range and FIG. 10 showsthat the LDP pad experiences breakthrough at about the same air velocityas the comparison pad.

These examples show that a multilayer pad having more layers than in theart provides a lower pressure drop, higher capacity, or both, incomparison with a separation device having a lower density and/orgreater thickness. Only a thicker set of conventional vanes provided alower pressure drop along much of the velocity range, yet breakthroughoccurred at a lower velocity for the vanes than for the novel LDP pad.

The foregoing description is meant to be illustrative and not limiting.Various changes, modifications, and additions may become apparent to theskilled artisan upon a perusal of this specification, and such are meantto be within the scope and spirit of the invention as defined by theclaims.

1. A knitted wire mesh mist collector pad having a low average density and comprising: at least three zones of flattened and crimped wire mesh tube, at least two of said zones having densities that are different from one another as a result of comprising meshes of different densities, and the zones arranged so that the density gradient in the direction of gas flow through the pad varies from low to high to low or from high to low to high.
 2. The mist collector pad of claim 1 wherein the lowest density zones are the first and last zones taken in the direction of the gas flow.
 3. The mist collector pad of claim 1 wherein at least one zone comprises wire mesh that has been crimped twice.
 4. The mist collector pad of claim 1 where the average density is 2.9 lb/ft³.
 5. A knitted wire mesh mist collector pad having a low average density and comprising at least four zones of flattened and crimped wire mesh tube, at least two of the zones having densities that are different from one another, the zones being arranged in the pad so as to produce a pattern that alternates between higher and lower density zones.
 6. The mist collector pad of claim 5 comprising five zones, the first and last zones taken in the direction of the gas flow having the lowest density meshes.
 7. The mist collector pad of claim 5 where the average density is 2.9 lb/ft³.
 8. A method for making a mist collector pad having a low average density comprising: providing predetermined cut lengths of first and second flattened, crimped, knitted wire mesh tubes having different densities from each other; layering one or more of said predefined cut lengths of said first tubes to define a first zone; layering one or more of said predefined cut lengths of said second tubes to define a second zone; layering one or more predefined lengths of said first tubes or of third tubes having a density different than said first and second tubes to define a third zone; and securing said first, second, and third zones in a grid suitable for installing in a process gas stream, wherein the respective densities of the meshes in the respective first, second, and third zones in the direction of the gas flow are lower, higher, and lower, or higher, lower, and higher.
 9. The method of claim 8 comprising layering to provide at least four zones with meshes having densities changing from lower to higher to lower to higher in the direction of the gas flow.
 10. The method of claim 8 comprising layering to provide at least five zones, the first and last zones each having a mesh with a density less than the densest mesh therebetween.
 11. The method of claim 8 comprising layering to provide at least six zones alternating between more and less dense meshes.
 12. The method of claim 8 comprising layering to provide an odd number of zones greater than or equal to five.
 13. The method of claim 8 wherein the mist collector pad has an average density of 2.9 lb/ft³.
 14. A process vessel comprising the mist collector of claim
 1. 